You may sometimes see these referred to as black onion seeds, but this tiny seed spice is quite unconnected to the onion and is better known as nigella or kalonji. They are the seeds of a herb which is closely related to the attractive garden plant 'love-in-a mist'.
India is its main producer and cooks there use it extensively in curries, pilaffs and pickles. Cooking brings out its flavour which is somewhat peppery and nutty and can be slightly bitter, so it should be used discreetly.
Having an elegant shape and jet black colour also makes it a popular decorative spice for tossing onto breads (such as naan) and savoury biscuits or pastries, as well as over salads.
It is one of the five spices, along with fennel, cumin, mustard and fenugreek seeds, that make up the Bengali spice blend 'panch phoran' - used to add flavour to lentil and other vegetarian dishes.